The present invention relates to a method for controlling the machining conditions between an electrode workpiece and an electrode wire stretched between two electrode wire guide members in a travelling wire EDM apparatus.
In travelling wire EDM apparatus, the relative motions of the electrode wire guide members and the workpiece which are effected such as to provide a cut in the workpiece along a predetermined path are usually controlled by a numerical control unit. At the present state of the art, the speed at which those relative movements are effected is controlled such as to maintain predetermined conditions of electrical discharges in the machining zone between the electrode wire and the electrode workpiece.
A conventional approach for avoiding deterioration of the machining, or electro-erosive electrical discharges, for example due to short circuits, consists in withdrawing rapidly the electrodes one from the other. Such a rapid withdrawal motion can be effected very quickly in cavity sinking operations in view of the rigid connection between the electrode tool and its corresponding servo mechanism and/or between the electrode workpiece and its corresponding servo mechanism. By contrast, such a rigid connection is not present in operations wherein the electrode workpiece is cut by a travelling electrode wire. This is due to the relative flexibility of the electrode wire between its guide members and to the forces exerted transversely on the electrode wire during machining. In effect, the position of the longitudinal axis of the wire relative to the axis of alignment of the wire guide members is dependent upon an equilibrium between the forces of repulsion due to the electrical discharges and the electromagnetic and electrostatic forces due respectively to the machining current and to the voltage, further complicated by the force exerted on the wire due to the traction applied to the wire by the wire guide members.
When a short circuit occurs, the machining electrical discharges are suppressed and the forces of repulsion disappear. The forces of attraction continue to be applied to the wire, which causes the wire to be attracted against the surface of the workpiece, thus tending to increase the short circuits and creating an undesirable unstable condition.
Swiss Pat. No. 476,544 discloses a structure for withdrawing the electrode wire by reversing the direction of displacement of the wire guide members as soon as a short circuit is detected. However, in order to effectively pull the electrode wire away from the workpiece it is necessary that the reverse displacement of the guide members be sufficient to create a pulling force higher than the forces of attraction. Consequently, the withdrawal of the wire is effected only after a substantial period of time has elapsed, which prevents an effective withdrawal of the wire through the motion of its guide members to be effected in an efficient manner, and thus prevents a rapid improvement of the machining conditions.
U.S. Pat. No. 4,104,502 proposes to improve the alignment of the wire between its guide members by increasing the force of attraction exerted on the wire by passing an auxiliary current through the wire in addition to the current required for machining the workpiece by electrical discharges. However, the auxiliary current acts electromagnetically upon the wire in a single direction, without allowing reversal of the direction of the force exerted on the wire because that force is created by an inducted magnetic field whose direction is reversed simultaneously with the direction of flow of the auxiliary current. A decrease in the intensity of the magnetic field by decreasing the auxiliary current causes a withdrawal of the wire away from the closed end of the cut in the workpiece, or machining front, only if some force of repulsion, due to the effect of machining electrical discharges, is still present. In addition, in the event of full short-circuits, it is no longer possible to pull the wire from the workpiece without moving the wire guide members which, as previously indicated, is a slow operation.